1. Field of the Invention
This invention relates to an apparatus for detecting the level of an object surface, and in particular to an apparatus useful for making the surface of a photosensitive substrate coincident with the focal plane of a projection optical system, for example, in precise photography
2. Related Background Art
In a projection exposure apparatus for the manufacture of semiconductor devices, as a device for detecting the focus position of a projection optical system, a system whereby an incident light is obliquely applied to a semiconductor wafer provided at a position whereat the image of a mask pattern is formed by a projection lens and the level of the surface of the semiconductor wafer is detected on the basis of the reflected light obliquely reflected from the surface of the semiconductor wafer is disclosed, for example, in Japanese Laid-Open Patent Application No. 56-42205 and U.S. Pat. No. 4,650,983 which is an improvement thereof.
According to this known position detecting system, a light beam is obliquely projected onto an object surface to form a slit-like optical image on the object surface and the reflected light therefrom is re-imaged on a detector constructed of a photoelectric conversion element. Correspondingly to a change in the level of the object surface, the position of the reflected optical image on the detector shifts on the detector. By detecting the amount of this shift, it is possible to determine whether the object surface is coincident with the focal plane of the projection lens.
However, when the surface position of a semiconductor wafer is to be actually detected by the use of the prior-art position detecting system constructed as described above, it has been found that there is a certain limit in the position detecting accuracy thereof. When the cause of it has been examined variously, it is often the case with the surface portion of a semiconductor wafer that a thin film such as photoresist adhered to a semiconductor substrate such as silicon, and when the thickness of the thin film has become the order of 1 to 2 .mu.m, interference is caused between the reflected light reflected by the surface of the thin film and the light transmitted through the surface of the thin film and reflected by the surface of the semiconductor substrate, and this is considered to cause a variation in the distribution of light intensity in a direction perpendicular to the optic axis of the reflected light. Incidentally, the light ray transmittance of a material formed of an organic substance, such as photoresist, is generally relatively high for a wavelength longer than the sensitizing wavelength (e.g., red light), and this has led to the problem that the reflected light from the front surface of the photoresist layer and the reflected light from the back surface thereof are liable to interfere with each other, thus causing an error.